Method and apparatus for storing and releasing heat by means of a phase change material

ABSTRACT

A method for storing and releasing heat by means of a phase change material. In said method, a phase change is caused in a first heat exchanging device ( 4 ) by supplying heat during a charging process in a storage medium comprising a phase change material in order to store the heat as latent heat in the storage medium, and a phase change is caused in the storage medium while heat is dissipated during a discharging process in the first or another heat exchanging device ( 2 ). The invention is characterized in that at least predominantly non-encapsulated phase change material is used as storage medium, the storage medium is fed to the first heat exchanging device ( 4 ) as a fluid stream or particle stream during the charging process and is discharged when the phase change has been completed, the storage medium is fed to the first or another heat exchanging device ( 2 ) as a fluid stream during the discharging process and is discharged from the heat exchanging device as a fluid stream or particle stream when the phase change has been completed, the storage medium is temporarily stored in a first storage tank ( 1 ) following the charging process and/or in the first or another storage tank ( 3 ) following the discharging process, and the storage medium is actively conveyed and heat is exchanged during the phase change as the charging process and/or the discharging process take/s place. An apparatus for storing and releasing heat by means of a phase change material is also provided.

BACKGROUND

The invention relates to a method for storing and releasing heat by aphase change material and also such an apparatus.

For a multiplicity of technical applications, storing heat is necessaryor advantageous. This concerns, for example, the use of renewableenergies in solar-thermal power plants and also cyclic processes inwhich the efficiency can be increased by storing surplus heat of a cyclefor use in a following cycle.

For efficient storage of heat or cold, in particular heat stores aresuitable which comprise a phase change material (PCM) as storage medium.Such latent heat stores have the advantage compared with other heatstores that large amounts of heat can be stored in a narrow temperaturerange. Compared with conventional sensible heat stores, using latentheat stores, high energy densities can be achieved with substantiallyconstant operating temperature. Thus, compared with conventional heatstorage using sensible heat, in typical latent heat stores, via atemperature change of 10 K in the phase change of the storage medium, aheat storage density that is ten to twenty fold higher can be achieved.The required amount of storage material and size of the correspondingapparatuses and containers are significantly reduced thereby.

A problem in the use of phase change materials is, in particular, thecomparatively low thermal conductivity of the organic or inorganicstorage media typically used (typically 0.5 to 1 W/(m K)). As a result,when the latent heat storage is implemented on an industrial scale, theproblem of inadequate heat transport between the storage medium and aheat transport fluid used for heat exchange arises. It is not only, aswith other storage systems, overcoming the heat transfer resistancesfrom the heat transport fluid (optionally via heat-exchange surfaces) tothe storage medium itself, but in addition overcoming comparatively highheat transfer resistances within the storage volume of the storagemedium in order to utilize the entire storage volume.

It is therefore known to improve the heat transport with the phasechange material by the phase change material being present inmicroencapsulation in a carrier liquid. In this case, typicallyparaffins are used as phase change material, which paraffins are presentin water as capsules having casings made of organic materials. In thiscase, the disadvantage of complex production of microcapsules occurs. Inaddition, by the use of paraffins and the organic materials, use of suchlatent heat stores is only possible below 100° C. Even if atemperature-stable encapsulation were to succeed, no suitable transportmedium seems to be available, since currently usual heat-transfer mediaare excluded: water because of excessive pressure; thermal oil becauseof expected vigorous reactions with the encapsulated salts and saltmelts, since then no encapsulation would be necessary.

In other previously known apparatuses, the phase change material isstationary. In this case it is known to encapsulate the phase changematerial in steel tubes which are flushed by heat transport fluid forthe heat exchange. Likewise it is known to form from the phase changematerial and a material having a comparatively higher thermalconductivity a composite material for developing a latent heat store asdescribed, for example, in US 2004/0084658 A1. It is additionally knownto increase the heat transfer surface area in the heat exchange bylamellae made of material having a high thermal conductivity which arein contact with the phase change material. For example, EP 1 816 176 A2discloses the use of graphite films for improving the thermal transportand heat transfer properties.

With the previously known apparatuses having a stationary phase changematerial, there is the disadvantage that due to the corrosive propertiesof the phase change materials typically used, large amounts ofhigh-value materials, such as corrosion-resistant steel, for instance,are required for forming the heat exchangers, and so in this case highcosts result.

SUMMARY

Therefore, the object of the invention is to provide a method forstoring and releasing heat by a phase change material and also toprovide such an apparatus which, compared with the prior art, are lesscomplex and therefore cheaper to implement. Furthermore, the methodaccording to the invention and the apparatus according to the inventionshould be usable in a broad temperature range, in particular in thetemperature range for the heat exchange of relevance for a multiplicityof applications between 100° C. and 300° C. or above.

This object is achieved by a method for storing and releasing heat by aphase change material and also an apparatus for storing and releasingheat by a phase change material. Advantageous embodiments of the methodaccording to the invention and of the apparatus according to theinvention as described in detail below.

The invention is based in the findings of the applicant that during thepreviously known methods and apparatuses, a costly and/or restrictivewith respect to the temperature range production of the storage mediumis necessary and/or direct coupling between the storage capacity on theone hand and the surfaces necessary for the heat exchange and therebycomplexity and size of the heat exchanger on the other exists. Thesedisadvantages are avoided in the method according to the invention andthe apparatus according to the invention in that there is a spatialseparation between heat exchange with the storage medium and storage ofthe storage medium, and in that phase change material is used inunencapsulated form.

In the method according to the invention, during a charging process(also termed charging process), in a storage medium which comprises aphase change material a phase change is caused in a heat-exchange devicewith addition of heat for storing the heat in the storage medium aslatent heat. In a discharging process, a phase change is caused in thestorage medium in a heat-exchange device with removal of heat.

It is important that the storage medium used is an at leastpredominantly unencapsulated phase change material. Furthermore, in themethod according to the invention and the apparatus according to theinvention, the storage medium is fed to the heat exchanger and thestorage medium is removed after the phase change has been completed, insuch a manner that there is a spatial separation between storage of thestorage medium and the site of the heat exchange.

In this case, during the charging process, the storage medium is fed asa fluid stream or as a particle stream to a first heat-exchange deviceand, after the phase change has been completed, the storage medium isremoved from the heat-exchange device as a fluid stream. During thedischarging process, the storage medium is fed as a fluid stream to thefirst heat-exchange device or a further heat-exchange device and afterthe phase change has been completed the storage medium is removed fromthe heat-exchange device as a fluid stream or as a particle stream. Theterm “fluid” in this case and hereinafter comprises substances in liquidand/or gaseous phase and/or mixtures of substances of liquid and gaseousphases.

The apparatus according to the invention for storing and releasing heatby a phase change material has a storage medium which comprises a phasechange material. Furthermore, the apparatus according to the inventioncomprises at least one first heat-exchange device and is constructed fora discharging process with release of latent heat of the storage mediumby a phase change in the first heat-exchange device and for a chargingprocess with storage of heat as latent heat in the storage medium by aphase change in the first heat-exchange device or a furtherheat-exchange device. It is important that the storage medium at leastpredominantly comprises unencapsulated phase change material, that inthe discharging process, the storage medium can be fed as a fluid streamto the first heat-exchange device, and after the phase change has beencompleted, the storage medium can be removed from the heat-exchangedevice as a fluid stream or as a particle stream, and also that in thecharging process, the storage medium can be fed to the firstheat-exchange device or a further heat-exchange device as a fluid streamor as a particle stream, and after the phase change has been completed,the storage medium can be removed from the heat-exchange device as afluid stream.

In the method according to the invention and the apparatus according tothe invention, therefore by feeding and removal of the storage medium toand from the site of the heat exchange, a spatial separation betweenheat exchange and storage of the storage medium is provided, in such amanner that, in particular, it is possible for the storage capacity tobe selectable as desired by provision, for example, of appropriatelydimensioned storage tanks for the storage medium, without an obligatorycoupling existing with respect to the dimensioning of the heat-exchangedevice. This results in a cost reduction, since the cost-intensiveelements of the heat-exchange device can be optimized, even in the caseof a large heat storage volume merely with respect to an optimum heatexchange.

By using unencapsulated phase change material, previously known PCMmaterials can be used, in particular for the temperature range relevantfor many applications between 120° C. and 300° C., likewise for highertemperatures, in such a manner that the temperature restrictionresulting when microencapsulated phase change materials are used isdispensed with. In the method according to the invention and theapparatus according to the invention, therefore, the storage medium isfed and removed directly to and from the heat-exchange device.

It is within the scope of the invention that the same heat-exchangedevice is used for the charging process and for the discharging process.Likewise, the use of a plurality of heat-exchange devices is possible.Preferably, at least two heat-exchange devices are provided, wherein thedischarging process is carried out in a first heat-exchange device andthe charging process in a second heat-exchange device spatiallyseparated therefrom. Hereinafter, the expression “heat-exchange device”denotes the device assigned to the respective process, regardless ofwhether one or more heat-exchange devices are provided for charging anddischarging processes, unless stated otherwise.

Furthermore, in the charging process, in the heat-exchange device duringthe phase change, active transport of the storage medium and heat supplyproceed simultaneously and/or during the discharging process in theheat-exchange device during the phase change, active transport of thestorage medium and heat removal proceed simultaneously. Correspondingly,in the apparatus according to the invention, at least the firstheat-exchange device is constructed in such a manner that, during thecharging process, in the heat-exchange device during the phase change,active transport of the storage medium and heat supply can be carriedout simultaneously.

In the method according to the invention and the apparatus according tothe invention, by the phase change in the heat-exchange device, the flowproperties of the storage medium change between the gaseous, liquidand/or solid phases. A further advantage of the invention thereforeresults from the fact that as described above, active transport of thestorage medium proceeds simultaneously to the phase change. Thenecessary transport of the storage medium through the heat-exchangedevice is therefore ensured at least during the active transport evenwhen the flow properties change owing to a phase change. Theheat-exchange device of the apparatus according to the inventiontherefore combines the properties of a transport device and theproperties of a heat exchanger.

The storage of the storage medium after the charging process and/orafter the discharging process proceeds in the method according to theinvention in at least one storage tank. This provides the spatialseparation between storage of the storage medium and site of the heatexchange, in such a manner that the storage volume is selectableindependently of the design of the heat-exchange device. Therefore, thisprovides a decoupling between storage of the storage medium on the onehand and design of the heat-exchange device, in particular dimensioningof the heat-exchange surface areas on the other. Correspondingly, theapparatus according to the invention comprises at least one storage tankfor receiving the storage medium after removal from the firstheat-exchange device and/or a further heat-exchange device.

In a preferred embodiment of the method according to the invention, thestorage medium is stored in a first storage tank after the dischargingprocess and in a separate second storage tank after the chargingprocess, and so the storage tanks can be optimized for the respectivephase state of the storage medium. Correspondingly, the apparatusaccording to the invention preferably comprises two storage tanks, forstorage of the storage medium in a first storage tank after carrying outthe charging process, and in a second storage tank after carrying outthe discharging process.

Likewise, the use of only one storage tank for storage of the storagemedium both after the charging process and after the discharging processis within the scope of the invention. In this case, preferably, recourseis made to separation of the storage medium regardless of the phase.Preferably, such a storage tank therefore has feed lines and outletlines at different heights, and so the storage medium can be suppliedand removed independently of the density and therefore independently ofthe phase.

In any case, the design of the storage tank or storage tanks asheat-insulated storage tanks is advantageous, in order to reduce heatexchange between the storage medium and the environment of the storagetank.

Storage tank and heat-exchange device are preferably connected byfluid-conducting lines.

Preferably, in the method according to the invention the storage mediumis actively transported during the charging process and/or dischargingprocess mechanically by motor-driven conveying means to theheat-exchange device and the heat exchange proceeds via the conveyingmeans, in particular the motor-driven elements thereof, and/or thestationary elements thereof. At least one heat-exchange device of theapparatus according to the invention preferably therefore has means forconveying the storage medium and is constructed in such a manner that,during the phase change, active transport of the storage medium and heatsupply and/or heat removal can be carried out simultaneously, inparticular by heat exchange via the conveying means, preferably themotor-driven elements thereof and/or stationary elements thereof.

A particularly efficient design of the method according to the inventionand the apparatus according to the invention results therefrom, sincethe motor-driven elements and stationary elements of the heat exchangerare directly in contact with the storage medium. Preferably, the heatexchange therefore proceeds at least via the motor-driven elements ofthe heat-exchange device that are directly in contact with the storagemedium. Thereby, the contact necessarily existing between themotor-driven elements and the storage medium is simultaneously used forheat exchange, and so no separate surfaces for heat exchange need to beprovided or they can at least be reduced.

A particularly advantageous design of the method according to theinvention and the apparatus according to the invention results from theactive transport of the storage medium during the charging and/ordischarging process by a screw conveyor. The screw conveyor comprises aconveyor screw having a screw thread arranged on a screw shaft and alsoa housing at least in part surrounding the conveyor screw. The housingsurrounds the conveyor screw at least in part and is preferablyconstructed so as to be cylindrical, covering the conveyor screw overthe entire periphery. Such screw conveyors are known per se for use inmethods and apparatuses outside this specialized field and aredescribed, for example, in DE 1653872 and DE 288663.

A screw conveyor has the advantage that the storage medium can betransported in different phases, in particular in the solid and liquidphases by a screw conveyor. Furthermore, the conveyor screw constructedso as to be able to be driven by motor of a screw conveyor has a largesurface area of the screw thread, which is directly in contact with thestorage medium, and so, in particular, the conveyor screw, in additionto the transport function, is simultaneously suitable as a heat-exchangeelement. This provides in a structurally simple design, the function notonly of transport but also of heat exchange. Preferably, moving and/ornon-moving elements of the screw conveyor have pathways for a heattransport fluid, for supplying or removing heat. In particular, it isadvantageous that the screw shaft of the conveyor screw of the screwconveyor has pathways for the heat transport fluid, wherein it ispreferred that the shaft of the conveyor screw is constructed as ahollow cylinder. This design of conveyor screw of a screw conveyor as ahollow screw is known in applications outside the specialist field andis described, for example, in DE 288663. Likewise, it is advantageousthat the housing of the screw conveyor has pathways for the heattransport fluid. The use of a screw conveyor simultaneously as conveyingmeans and as heat-exchange device in the case of latent heat storesleads to surprisingly structurally simple designs of the apparatusaccording to the invention and makes possible, in a simple manner, thespatial separation between storage of the storage medium and the site ofheat exchange.

In the method according to the invention, the storage medium istransported directly. Preferably, the storage medium therefore does notcomprise a carrier medium, such as water, for example, i.e. preferablyno carrier medium is used for transporting the storage medium.

The method according to the invention and the apparatus according to theinvention are not restricted to certain phase transitions. Thus, the useof the method and the apparatus for phase change materials and attemperature ranges at which during the discharging process a phasechange proceeds from gaseous to liquid and during the charging process aphase change proceeds from liquid to gaseous is within the scope of theinvention. Particularly advantageously, the method according to theinvention and the apparatus according to the invention are designed,however, in such a manner that during the discharging process thestorage medium is supplied in liquid form, a phase change from liquid tosolid proceeds and the storage medium is removed as a particle streamand that during the charging process, the storage medium is supplied asa particle stream, a phase change from solid to liquid proceeds, and thestorage medium is removed in liquid form.

The solid/liquid phases of the phase change material have the advantagethat a simpler transport of the storage medium in these phases can becarried out. In particular, when the heat-transfer device is embodied asa screw conveyor, transport of the storage medium is possible in asimple manner either as a particle stream or else in liquid form.

Preferably, in the method according to the invention, during thedischarging process in the heat-exchange device during and/or after thephase change of the storage medium into the solid phase, comminution ofthe storage medium to particles takes place. Correspondingly, at leastthe first heat-exchange device of the apparatus according to theinvention preferably has comminution means and is designed forcomminution of the storage medium to particles during and/or after thephase change. This ensures that the storage medium is transportable as aparticle stream even after the phase change to the solid phase.

In particular, in the preferred design of the heat exchanger as a screwconveyor, by structural design of the conveyor screw and/or by design ofthe surface of the conveyor screw, comminution of the storage medium toparticles can be effected. In particular, it is advantageous toconstruct the screw conveyor with a plurality of adjacently arrangedconveyor screws having parallel screw shafts. The screw conveyor in thiscase is designed in such a manner that at times a mutual erosion of theflanks of the conveyor screws proceeds, in such a manner that adhesionof storage material is prevented. In this case, recourse can be made topreviously known structural designs in which the erosion is achieved bytemporary change of the speed of rotation of at least one screw. This isdescribed in DE 1553134 in the embodiment having two counter-rotatingconveyor screws, of which one conveyor screw is constructed so as to beright handed and the other conveyor screw is constructed so as to beleft handed. Likewise, the erosion can be achieved in a manner known perse in the case of co-rotating and identically-handed conveyor screws,the flanks of which have a spacing in the central position, by changingthe speed of rotation of one or both conveyor screws, as described, forexample, in DE 1653872.

Likewise, it is within the scope of the invention to provide separatecomminuting means, such as, for example, mutually engaging gear-likecomminution tools, blade-like tools, choppers or other elements forcomminuting the storage medium in the heat-exchange device.

Preferably, the first heat-exchange device is constructed in such amanner that the storage medium is in granular form after the dischargingprocess and optionally additional comminution.

The design of the heat-exchange device as a screw conveyor isparticularly advantageous, as described hereinbefore. In particular forthe discharging process, the use of a screw conveyor is advantageous onoccurrence of the phase transition from liquid to solid. Therefore,preferably, at least the heat-exchange element used for the dischargingprocess has a screw conveyor. Likewise, the design of the heat-exchangedevice as per other conveying means, such as structural designs that areknown per se, for example, of pumps, in particular an apparatus havinggear-shaped mutually engaging rolls which are arranged transversely tothe transport direction of the storage medium, or differently designedgear pumps, is in the scope of the invention.

A particularly efficient storage and release of heat results from apreferred embodiment of the method according to the invention and theapparatus according to the invention in that, during the chargingprocess, heat is additionally fed to the storage medium after the phasechange for additional storage of sensible heat by the storage medium andcorrespondingly during the discharging process, sensible heat is removedfrom the storage medium before the phase change. Therefore, not only thestorage capacity for latent heat but in addition also the storagecapacity for sensible heat of the storage medium is utilized thereby.The supply and removal of sensible heat proceeds in a temperature rangein which no phase change of the storage medium proceeds.

The supply and removal of sensible heat preferably proceeds via the sameheat exchanger, via which the latent heat is supplied to and removedfrom the storage medium. As a result, no additional heat exchangers arenecessary.

In a further preferred embodiment of the method according to theinvention and the apparatus according to the invention, the sensibleheat is supplied and removed by one or more additional heat exchangers,wherein the heat exchanger in the charging process is connecteddownstream of the heat-exchange device for storage of the latent heatand in the discharging process, is connected upstream of theheat-exchange device for removing the latent heat. By this meansoptimization of the respective heat exchanger for transfer of the latentor sensible heat is possible.

The method according to the invention and the apparatus according to theinvention are preferably constructed for a temperature range in the heatexchange between 100° C. and at least 350° C., since a multiplicity oftypical applications are in this temperature range. Equally,applications are known at which higher temperatures, in particulartemperatures up to 500° C., are advantageous. These higher temperatureranges are also within the scope of the invention when an appropriatestorage medium is selected. Advantageously, the method according to theinvention and the apparatus according to the invention are thereforeconstructed for temperatures in the range between 100° C. and 500° C. Inparticular, in the case of additional storage of sensible heat asdescribed above, higher temperatures, for example in the range between100° C. and 500° C., are also within the scope of the invention. Theabove-mentioned temperature ranges are achievable, in particular, byusing salts as storage medium.

The heat exchange during charging and discharging processes is possiblein principle in a manner known per se. In particular, the supply and/orremoval of heat using a heat transport fluid, preferably using a heattransport gas or a heat transport liquid or a mixture of liquid and gasis within the scope of the invention. In particular, the use of thermaloil, water, steam or a water-steam mixture (saturated steam) as heattransport fluid is advantageous. Likewise, the supply and/or removal ofheat in other ways, for example using radiation, is within the scope ofthe invention, in particular the leading of the storage material throughthe absorber of a solar collector during the charging process.

The use of a heat transport fluid for the heat exchange in the chargingprocess and/or the discharging process is in particular advantageous. Aparticularly structurally simple design results from the use of a liquidsuch as, for example, thermal oil or water, as heat transport fluid.

In a further preferred embodiment of the method according to theinvention and the apparatus according to the invention, during thedischarging process, a phase change of the heat transport fluid fromliquid to gaseous is caused by the heat given off from the storagemedium to the heat transport fluid and/or, during the charging process,a phase change of the heat fluid from gaseous to liquid is caused due tothe heat given off from the heat fluid to the storage medium. By thismeans, a particularly efficient transfer and transmission of the storedheat energy of the latent heat store is achieved. In particular, theabove-mentioned preferred embodiment is advantageous on use of theapparatus according to the invention and the method according to theinvention in combination with turbines driven by the heat-transferfluid. A further increase in efficiency in the heat transfer using theheat transport fluid is achieved in a preferred embodiment, in that,during the discharging process, the heat transport fluid is firstvaporized using the heat released by the storage medium and then thevaporous heat transport fluid is additionally superheated using the heatreleased by the storage medium.

As mentioned hereinbelow, in particular the construction of theheat-exchange device as a screw conveyor is advantageous. Preferably, inthis case, the heat carrier fluid flows through the motor-drivenconveyor screw for heat exchange. In particular, it is advantageous thatthe heat fluid flows for heat exchange through not only the conveyorscrew, but also the stationary elements of the screw conveyor which arein direct contact with the storage medium, in particular the housing, inparticular preferably according to the counterflow principle.

Likewise, the configuration of the heat exchanger according to otherpreviously known embodiments of heat exchangers such as, for example, asan entrained-flow heat exchanger or as emitter or absorber for radiationis within the scope of the invention.

Likewise, the supply of heat to the heat-exchange device in the chargingprocess using a heat-transfer fluid in the vaporous, gaseous or liquidform or by means of a mixture comprising a plurality of phases is in thescope of the invention, likewise supplying heat using radiation, inparticular solar radiation.

Preferably, the storage medium consists exclusively of phase changematerial. This gives an inexpensive configuration of the method and ofthe apparatus.

An increase in efficiency of the method according to the invention andthe apparatus according to the invention is advantageously achieved inthat the storage medium, in addition to the phase change material,comprises particles having a thermal conductivity greater than that ofthe phase change material. By this means, the overall thermalconductivity of the storage medium is increased and therefore theefficiency in the heat exchange is increased not only in the chargingprocess but also in the discharging process. Particularly suitableparticles in this case are nanoparticles or microparticles made ofgraphite. Preferably, the proportion of these particles of the totalvolume of the storage medium is below 10 percent by volume.

A further increase in the efficiency of the method according to theinvention and the apparatus according to the invention is advantageouslyachieved in that admixtures are added to the storage medium which favorthe formation of granules on transition from the liquid to the solidphase, or prevent the solidification of the entire volume, i.e.formation of a high-volume solid phase.

In the method according to the invention and the apparatus according tothe invention, it is possible to use materials that are known per se asphase change material. Materials which are suitable in particular aresalt systems, preferably binary nitrate salts, nitrate salt mixtures, inparticular comprising one or more substances of the group KNO₃, NaNO₃,KNO, KNO₃—NaNO₃, KNO₃—LiNO₃. The temperature range between 100° C. and500° C. which is relevant in typical applications is covered thereby.

In the method according to the invention and the apparatus according tothe invention, at least one active transport of the storage mediumproceeds during the phase change in the charging process and/or thedischarging process. Preferably, the apparatus according to theinvention comprises one or more additional transport means which are notdesigned as a heat-exchange device and are arranged downstream and/orupstream of the heat-exchange device in the transport path of thestorage medium. This ensures fault-free transport of the storage medium.In particular, it is advantageous to provide at least one additionaltransport means in the transport path of the storage medium in solid,granular form, since the transport of the granular storage medium iscomparatively more susceptible to interference such as compacting of thestorage medium compared with the transport of the storage medium inliquid or gaseous phase.

The method according to the invention and the apparatus according to theinvention are usable, in particular, for the thermal storage of heatenergy, as a thermal store for solar-heating power plants, in particularin direct steam generation, or as a thermal store for process heatapplications.

Preferably, the storage medium comprises exclusively non-encapsulatedphase change material.

The supply and/or removal of the storage medium as a particle stream inthe advantageous configurations described previously preferably proceedby transporting the storage medium in granular form, in particularwithout a carrier fluid, preferably without a carrier liquid, for theparticles.

The transport means of the heat-exchange devices are preferablyconstructed of heat-conducting material, in particular of steel,preferably stainless steel. Likewise, the use of ceramic materials iswithin the scope of the invention, wherein, however, they cause highermaterial costs compared with steel.

The apparatus according to the invention is preferably designed forcarrying out the method according to the invention or an advantageousembodiment thereof. Likewise, the method according to the invention ispreferably designed for being carried out using an apparatus accordingto the invention or an advantageous embodiment thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Further preferred features and embodiments will be described hereinafterwith reference to the figures and the exemplary embodiments. In thefigures

FIG. 1 shows the schematic representation of a first exemplaryembodiment of an apparatus according to the invention for storing andreleasing heat using a phase change material, wherein the apparatus ispart of an apparatus for converting solar irradiation into electricalenergy and comprises two storage tanks and also two heat-exchangedevices and

FIG. 2 shows the schematic representation of a second exemplaryembodiment of an apparatus according to the invention for storing andreleasing heat using a phase change material which is a modification ofthe first exemplary embodiment and has only one storage tank and alsoonly one heat-exchange device.

The apparatus of the first exemplary embodiment according to FIG. 1comprises a first storage tank 1 which contains liquid storage medium.The first storage tank 1 has thermal insulation, such that only slightheat exchange with the environment takes place. The storage mediumconsists of the phase change material NaNO₃.

When the storage medium is stored in storage tank 1, the temperature ofthe storage medium is above the melting point of the storage medium,i.e. in this case above 308° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus shown in FIG. 1 further comprises a first heat-exchangedevice 2 which is fluid-conductingly connected via a line 2 a to thestorage tank 1. The heat-exchange device 2 comprises a screw conveyor inwhich a conveyor screw that is rotatably mounted and driven using amotor is arranged within a cylindrical housing. The shaft of theconveyor screw is constructed as a hollow cylinder and isfluid-conductingly connected to a heat transport fluid circuit.Likewise, in the shell of the cylindrical housing for the conveyorscrew, lines are arranged which are fluid-conductingly connected to thecircuit for a heat transport fluid.

On the exit side of the screw conveyor is arranged a comminution unitwhich in turn is connected via a tubular line 3 a to a second storagetank 3.

The external circuit for the heat transport fluid is indicated in FIG. 1at the heat-exchange device 2 by arrows.

For carrying out the discharging process, thus, using a control unit(which is not shown) the conveyor screw of the screw conveyor of theheat-exchange device 2 is set rotating about the shaft designed as ahollow cylinder, and so, using the screw conveyor, storage medium istransported out of the storage tank 1 into the heat-exchange device 2.At the same time, via a pump device that is not shown, supply andremoval of heat transport fluid proceeds according to the abovementionedarrows, and so heat transport fluid flows through the heat transportfluid circuit of the heat-exchange device 2. In this case, on the intakeside of the screw conveyor, owing to the lower temperature of thesupplied heat transport fluid, compared with the storage medium, thestorage medium is cooled, which leads to a phase change of the storagemedium in the transport screw of the heat-exchange device 2. The heatreleased during the phase change is released via the conveyor screw ofthe screw conveyor and also the cylindrical housing of the screwconveyor to the heat transport fluid flowing through these elements, andthus the latter is heated. In the exemplary embodiment shown in FIG. 1,the heat transport fluid is implemented as water which is supplied tothe heat-exchange device 2 in liquid form having a temperature below theboiling point valid for the prevailing pressure. Due to the heating inthe screw conveyor on account of the released latent heat of the storagemedium, the heat transport fluid undergoes a phase change, and so itleaves the first heat-exchange device 2 in the form of steam. The steamis converted into electrical energy by means of a turbine in a furtherunit which is not shown.

In the screw conveyor of the heat-exchange device 2, therefore a phasechange of the storage medium proceeds from liquid to solid. On accountof the transport of the storage medium during the phase change in thescrew conveyor, however, a uniform solid body does not form, rather, dueto the constant further transport, storage medium particles of differentsizes develop. On the exit side of the screw conveyor, in theheat-exchange device 2, the comminution device is arranged in such amanner that said particles, after exit from the screw conveyor, passinto the comminution device and are there comminuted into smallerparticles in such a manner that the storage medium is present ingranular form.

Via the tubular connection 3 a, the particle stream of the storagemedium passes into the second storage tank 3. The second storage tank 3is likewise formed in a heat-insulated manner. The temperature of thestorage medium in the storage tank 3 is in the range between the ambienttemperature and the melt temperature of the storage medium, i.e. in thiscase below 308° C.

For carrying out the charging process, via a further tubular connection4 a, the storage medium is supplied from the second storage tank 3 as aparticle stream to a second heat-exchange device 4.

The second heat-exchange device 4 likewise comprises a screw conveyorwhich is fundamentally identical in structure to the screw conveyordescribed for the heat-exchange device 2.

The second heat-exchange device 4 in this case is arranged below thestorage tank 3 and the tubular connection 4 a opens in the bottom regionof the storage tank 3 in such a manner that using a motor-driven slidevalve arranged in the tubular connection 4 a, by means of control viathe aforesaid control unit, in a simple manner, the particle stream fromthe storage tank 3 into the second heat-exchange device 4, and therebyon the intake side to the screw conveyor of the second heat-exchangedevice 4, is controllable.

Also, the screw conveyor of the second heat-exchange device 4, not onlyin the conveyor screw, but also in the cylindrical housing, has pathwaysof a circuit for a heat transport fluid. These are fluid-conductinglyconnected to a further external circuit for a heat transport fluid, asindicated by the arrows for the heat-exchange device 4 in FIG. 1.

The second heat transport fluid is also implemented as water. Thecircuit of the second heat transport fluid is connected to asolar-heating apparatus in which the heat transport fluid is vaporizedby solar irradiation.

Correspondingly, in the second heat-exchange device 4, a feeding,characterized by the arrows, of steam into the circuit for the heattransport fluid of the screw conveyor proceeds, in such a manner that aphase change from solid to liquid of the storage medium transported inthe screw conveyor during the transport proceeds owing to the heatsupplied by the steam and simultaneously, owing to the cooling of thesteam supplied, the heat transport fluid condenses to water. The heattransport fluid is therefore passed in liquid form from the secondheat-exchange device 4 and to the solar-heating apparatus.

The screw conveyor of the second heat-exchange device 4 isfluid-conductingly connected to the first storage tank 1 on the exitside via a line 1 a. By means of the screw conveyor of the secondheat-exchange device 4, therefore the liquid storage medium istransported after the charging process into the first storage tank 1.

The screw conveyors of the first heat-exchange device 2 and secondheat-exchange device 4 are each constructed of corrosion-resistantsteel, in such a manner that firstly destruction via the corrosiveproperties of the storage medium does not take place and secondly, owingto the high thermal conductivity of the steel, good heat exchange isensured between the heat transport fluid and the storage medium.

By the spatial separation of the storage of the storage medium in thestorage tanks 1 and 3 on the one hand, and of the heat exchange in theheat-exchange devices 2 and 4, on the other, the dimensions of the screwconveyors are optimized in each case for an optimum heat exchange undera predetermined conveyor speed. Independently thereof, the volume of thestorage tanks 1 and 3 can be selected as desired, depending on how highthe demand for heat storage capacity is.

In the exemplary embodiment shown in FIG. 1 when a 50 MWel turbine isused, the two storage tanks 1 and 3 each comprise a volume of about 800m³/h of storage capacity. That means, for a store having, for example,7.5 h storage capacity, tanks each of 6,000 m³ volume are necessary. Incomparison therewith, currently designed salt melt stores based onsensible heat require tanks each having a volume of 14,000 m³.

The particle diameter of the particles of the storage medium aftercomminution in the first heat-exchange device 2 is in the range betweenabout 1 mm and 10 mm.

The transport capacity of the screw conveyors of the first heat-exchangedevice 2 and second heat-exchange device 4 is 500 kg/s for providingheat for a 50 MW turbine.

The apparatus of the second exemplary embodiment according to FIG. 2comprises only one storage tank 11 which contains both liquid and solidstorage material in granular form. Furthermore, the apparatus shown inFIG. 2 comprises only one heat-exchange device 12. Where not statedotherwise hereinafter, the storage tank 11 is constructed in accordancewith the above-described storage tank 1 and the heat-exchange device 12according to the above-described heat-exchange device 2.

In contrast to the first exemplary embodiment shown in FIG. 1, thestorage tank 11 and the heat-exchange device 12 of the second exemplaryembodiment each have two supply and removal lines for the storagemedium.

The storage tank 11 is fluid-conductingly andparticle-stream-conductingly connected to the heat-exchange device 12via an upper line 11 a and a lower line 11 b. On account of thediffering density of the storage material in liquid and solid granularform, in the storage tank 11, a spatial separation results between thetwo phases, in such a manner that by means of the upper line 11 a,storage medium can be supplied in liquid form and by means of the lowerline 11 b, storage medium can be supplied in solid granular form to theheat-exchange device 12.

The charging and discharging processes correspond fundamentally to thoseof the first exemplary embodiment:

For carrying out the charging process, storage medium is supplied insolid, granular form via the lower line 11 b to the heat-exchange device12. In the heat-exchange device 12 constructed as a screw conveyor, viaa pump device that is not shown, a supply and removal of heat transportfluid is carried out according to the arrows 12 b. A heat transfer fromthe heat transport fluid to the storage medium proceeds thereby in theheat-exchange device 12, in such a manner that during the transport ofthe storage medium through the conveyor screw, a phase change from solidto liquid proceeds. The liquid storage medium is returned to the storagetank via the line 12 c which opens out in the upper region of thestorage tank 1.

For carrying out the discharging process, liquid storage material issupplied via the upper line 11 a to the heat-exchange device 12 and, bymeans of a pump device that is not shown, heat transport fluid issupplied and removed according to the arrows 12 a, in such a manner thatheat release proceeds from the storage medium to the heat transportfluid and a phase change of the heat transport fluid from liquid tosolid proceeds during transport through the conveyor screw in theheat-exchange device 12.

The solid storage material which, similarly to the apparatus shown inFIG. 1, is comminuted by means of a comminutor into a granular form, isreturned to the storage tank via the line 12 d, which opens out in thelower region of the storage tank.

The apparatus according to FIG. 2 has the advantage that only onestorage tank and only one heat-exchange device are necessary.

In the apparatuses shown in FIG. 1 and FIG. 2 in each case the controlof the charging process and discharging process proceeds via a controlunit which is connected in particular to motor drives of the conveyorscrews and also to corresponding motor-actuatable valves and slidevalves at the respective exits of the storage tanks for controlling thetransport of the storage medium.

1. A method for storing and releasing heat by a phase change material,wherein in a charging process causing, a phase change in a storagemedium which comprises a phase change material by addition of heat in afirst heat-exchange device (4, for storage of the heat in the storagemedium as latent heat, and in a discharging process, in the firstheat-exchange device (2) or another heat-exchange device (2), causing aphase change in the storage medium with removal of heat, the storagemedium used is at least predominantly unencapsulated phase changematerial, and during the charging process, feeding the storage medium tothe first heat-exchange device (4) as a fluid stream or as a particlestream and, after the phase change has been completed, removing thestorage medium, during the discharging process, feeding the storagemedium as a fluid stream to the first heat-exchange device (2) oranother heat-exchange device (2) and, after phase change has beencompleted, removing the storage medium from the heat-exchange device asa fluid stream or as a particle stream, temporarily storing the storagemedium, after at least one of the charging process or the dischargingprocess in a first storage tank (1), or temporarily storing the storagemedium after the charging process in the first storage tank and afterthe discharging process in another storage tank (3), and during at leastone of the charging process or during the discharging process, duringthe phase change, at the same time proceeding with active transport ofthe storage medium and the heat exchange.
 2. The method as claimed inclaim 1, wherein the active transport during at least one of thecharging process or discharging process proceeds mechanically by amotor-driven transport of the heat-exchange device (2, 4) and the heatexchange proceeds at least via at least one of motor-driven elements orstationary elements of the motor driven transport.
 3. The method asclaimed in claim 2, wherein the active transport during the chargingprocess or discharging process proceeds via a screw conveyor.
 4. Themethod as claimed in claim 1, wherein during the discharging process,the storage medium is fed in liquid form, a phase change from liquid tosolid proceeds, and the storage medium is removed as the particlestream, and during the charging process, the storage medium is fed asthe particle stream, a phase change from solid to liquid proceeds, andthe storage medium is removed in liquid form.
 5. The method as claimedin claim 4, wherein during the discharging process the storage medium iscomminuted at least one of during or after the phase change to theparticles in the heat-exchange device (2).
 6. The method as claimed inclaim 5, wherein during the charging process, heat is additionally fedto the storage medium after the phase change for additional storage ofsensible heat, and during the discharging process, the sensible heat isremoved from the storage medium before the phase change.
 7. The methodas claimed in claim 1, wherein during at least one of the chargingprocess or the discharging process, the heat exchange proceeds by a heattransport fluid.
 8. The method as claimed in claim 7, wherein during atleast one of the charging process or discharging process, a phase changeof the heat transport fluid is caused.
 9. The method as claimed in claim1, wherein the storage medium has particles having a thermalconductivity greater than that of the phase change material.
 10. Themethod as claimed in claim 1, wherein the storage medium includesadmixtures that at least one of favor development of granules ontransition from the liquid phase to the solid phase, or prevent thedevelopment of a high-volume solid phase.
 11. An apparatus for storingand releasing heat by a phase change material, comprising a storagemedium which comprises a phase change material, and at least one firstheat-exchange device (2), constructed for a discharging process withrelease of latent heat of the storage medium by a phase change in thefirst heat-exchange device (2) and for a charging process with storageof heat as latent heat in the storage medium by a phase change in thefirst heat-exchange device or further heat-exchange device (4), thestorage medium comprises at least predominantly unencapsulated phasechange material, and the apparatus is formed in such a manner that,during the charging process, the storage medium is fed to the at leastone first heat-exchange device as a fluid stream or as a particle streamand after the phase change has been carried out, the storage medium isremoved, during the discharging process, the storage medium is fed as afluid stream to the first heat-exchange device (2) or furtherheat-exchange device (2) and after the phase change has been carriedout, the storage medium is removed from the heat-exchange device as afluid stream or as a particle stream, the at least one heat-exchangedevice (2, 4) has a transport device to transport the storage medium andis formed in such a manner that, during the phase change, activetransport of the storage medium and heat exchange are carried outsimultaneously, and the apparatus comprises at least one storage tank(1, 3) for receiving the storage medium after removal from the at leastone first heat-exchange device (2, 4) or the further heat-exchangedevice (2, 4).
 12. The apparatus as claimed in claim 11, wherein atleast the transport device of the first heat-exchange device (2)comprises a motor drive and a driven element for transporting thestorage medium during the charging process, and at least one of amotor-driven element or a stationary element of the transport device isconstructed for the heat exchange.
 13. The apparatus as claimed in claim12, wherein at least the first heat-exchange device (2) comprises ascrew conveyor.
 14. The apparatus as claimed in claim 13, wherein thefirst heat-exchange device (2) comprises a comminuter that comminutesthe storage medium to particles at least one of during or after thephase change.
 15. (canceled)
 16. The method as claimed in claim 3,wherein the screw conveyor has at least one hollow screw.
 17. The methodas claimed in claim 8, wherein during the discharging process, a phasechange of the heat transport fluid from liquid to gaseous is caused byheat given off from the storage medium to the heat transport fluid. 18.The method of claim 8, wherein during the charging process, a phasechange of the heat transport fluid from gaseous to liquid is caused bythe heat given off from the heat transport fluid to the storage medium.19. The apparatus of claim 12, wherein each of the heat-exchange devicescomprises in each case one conveyor screw.